Wireless local area networks (WLANs), such as those conforming to the IEEE 802.11 family of standards, have become pervasive in recent years, and are increasingly used for both data communication and voice-over-IP (VOIP) communication. Such wireless networks operate in various frequency bands, including bands in the 5 GHz region that are popularly called DFS bands.
Applicable regulations require that, on DFS bands, if a mobile device hears a radar on its frequency, it must vacate the frequency in a given amount of time (generally about 10 seconds) and must remain off of the frequency for a certain length of time (approximately 30 minutes). Furthermore, no device can transmit on the frequency until it learns that no radar is present on that frequency. The applicable regulations allow slave devices that are enabled to transmit on the frequency by master devices. The master devices then assume responsibility for listening for radar and informing slave devices that they must switch to a frequency that has no radar interference.
In accordance with 802.11, the method that master devices (i.e., access points) use to enable slave devices utilizes the beacon. If there is no channel switch announcement present in the beacon, then the slave device can transmit on the channel. When the access point adds the channel switch announcement to the beacon, then the mobile device cannot subsequently transmit on that frequency.
If a device is engaging in a voice call, perhaps using G711 to encode voice packets, the device transmits a voice packet to the access point every 20 ms, and then polls the access point and receives back from the AP a G711 voice packet. The traditional method for learning about an access point on a DFS frequency is to listen for a duration no greater than the beacon interval (100 ms) on that frequency. If the device leaves the frequency of the access point to which it is currently associated and transfers to a DFS channel to listen for an (unknown) access point, it is likely that up to five voice packets will be delayed in being transmitted to the access point, and up to five voice packets will be delayed from reception from the access point. This is highly undesirable, as it results in, among other things, poor voice quality.
Accordingly, it is desirable to provide improved methods and systems for passive scanning in WLANs. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.